It is well known that echography and NMRI are investigative diagnosis techniques which enable the direct electronic visualization of internal organs in living beings and are therefore powerful help and guide in prognosis, medical treatment and surgery. These techniques can often advantageously supplement or replace X-ray tomography as well as the use of radio-active tracer compounds which may have obvious undesirable side-effects.
It is also well known that contrast echography relies on the administration to patients of dispersions or suspensions of microbodies containing air or a gas, in a medium, and thereafter applying ultrasonic waves which become reflected by said microbodies to provide desired echographic signals. In this connection, it has been recognized that air or gas-filled microspheres, e.g. microbubbles or microballoons, suspended in a liquid are exceptionally efficient ultrasound reflectors for echography. In this disclosure the term of "microbubble" specifically designates air or gas globules in suspension in a liquid which generally results from the introduction therein of air or a gas in divided form, the liquid preferably also containing surfactants or tensides to control the surface properties and the stability of the bubbles. The term of "microcapsule" or "microballoon" designates preferably air or gas bodies with a material boundary or envelope, e.g.a polymer membrane wall. Both microbubbles and microballoons are useful as ultrasonic contrast agents. For instance injecting into the blood-stream of living bodies suspensions of gas microbubbles or microballoons (in the range of 0.5 to 10 .mu.m) in a carrier liquid will strongly reinforce ultrasonic echography imaging, thus aiding in the visualization of internal organs. Imaging of vessels and internal organs can strongly help in medical diagnosis, for instance for the detection of cardiovascular and other diseases.
It is also well known that NMRI techniques comprise subjecting a patient to a main static magnetic field combined with a linear gradient magnetic field, both being directed to some parts of the body to be investigated. The magnetic fields act on the nuclei of atoms with fractional spin quantum numbers and encode them into various degrees of statistical alignment with different resonant frequencies in a selected direction of orientation; the nuclei of concern here are mainly that of hydrogen atoms, i.e. protons, these being predominantly that of molecules present in relatively high concentration in or around the organs to be investigated, viz, the protons of water and lipids. For doing the measurements, one will apply to the parts of the body under investigation pulses of radio-frequency that matches with the resonance energy of the protons involved in the tissues or fluids of said parts of the body. When the protons under consideration are excited by a pulse of resonant energy, they are raised to a higher energy state which causes them to flip from the average orientation direction controlled by the magnetic field. Thereafter, the protons will return to their original state by relaxation in an exponential time dependent fashion, the corresponding energy then reemitted (spin-echo) forming a response signal typical of the protons under consideration, i.e. depending on their immediate environment.
NMRI techniques are actually based on the detecting, acquiring and electronically processing of this signal (according to Fourier transforms) and thereafter displaying it spatially on a screen, thus forming an image whose various patterns correspond to areas having protons in different environments, i.e. to protons belonging to organ tissues or body fluids being subjected to investigation.
Among the critical factors pertaining to MRI, one usually distinguishes two mutually perpendicular components of the proton-distinctive relaxation time parameter, namely the spin-lattice component along the axis of magnetization (called T.sub.1), which corresponds to the release of energy to the nuclear environment, and the perpendicular or transverse (spin-spin) relaxation component (called T.sub.2), that corresponds to the returning of the nucleus to the initial statistical energy level. Either T.sub.1 or T.sub.2 can contribute to the definition of the NMR images depending on the kind of organ selected and the measurement conditions.
It should be noted that when the measurements are carried out in the absence of agents added for increasing image contrasts, the differences in relaxation time constants between protons in various parts of the organs are small and the image is of poor to bad quality. The contrast effect can however be enhanced by the presence, in the environment of the hydrated molecules under excitation, of a variety of magnetic species, e.g. paramagnetic (which mainly affect T.sub.1) and ferromagnetic or superparamagnetic (which mainly affect the T.sub.2 response). The paramagnetic substances include some metals in the ionic or organo-metallic state (e.g. Fe.sup.+3, Mn.sup.+2, Gd.sup.+3 and the like, particularly in the form of chelates to decrease the intrinsic toxicity of the free metal ions). Ferromagnetic contrast substances preferably include magnetic aggregate particles of micronic or submicronic size, i.e. not smaller than about 100-200 nm, for instance particles of magnetite (Fe.sub.3 O.sub.4), .gamma.-Fe.sub.2 O.sub.3, ferrites and other magnetic mineral compounds of transition elements. Superparamagnetic materials are usually very small magnetic particles (blow abut 100-150 nm) which, because their size is under a critical value, do not behave any longer as small autonomous magnets, i.e they will align in a preferential direction only when subjected to an external magnetic field. The advantage of the superparamagnetic materials (also defined sometimes as superparamagnetic fluids) over the ferromagnetic particles is mainly of efficiency density, i.e. being smaller, the number of available magnetic particles for a given weight of metal is greater in the case of superparamagnetic particles than with ferromagnetic particles and the magnetic efficiency on the neighboring protons is further enhanced.
For ultrasonic or NMRI imaging of the digestive tract, the particulate contrast agents, whether in the form of gas-filled microspheres or particles of ferromagnetic, superparamagnetic, or paramagnetic materials, are usually administered orally or rectally, either neat or preferably with a carrier.
For instance, EP-A-275.215 (AMERSHAM) discloses NMRI contrast enhancers for the investigation of the digestive tract comprising complexes of paramagnetic metal species like gadolinium, iron, manganese and the like associated with mineral particulate carriers such as alkaline-earth polyphosphates and apatite.
EP-A-83.760=WO85/05534 (AMERSHAM) discloses EDTA, DTPA and NTA chelates of paramagnetic metals chemically bonded to organic polymer carriers such as sepharose, dextran, dextrin, starch and the like.
Also in EP-A-299.920 (SCHERING), there are disclosed complexes between paramagnetic metals such as Cr, Mn, Fe, Ni, Co, Gd, etc. and polysulfated oligosaccharides like sucrose or maltose, these complexes being used for NMRI of the digestive tract.
It has been indicated above that paramagnetic contrast agents in which the metals are in the ionic state or in the form of metal-organic compounds are often metabolizable and toxic and, although this toxicity can be controlled to some extent by using very strong chelatants and non-metabolizable polymer carriers, it is desirable to further minimize possible hazards by using less toxic materials, e.g. non-metabolizable magnetic particles of sufficient size not to diffuse through the intestinal membrane; the micronic ferromagnetic and nanometric superparamagnetic aggregate particles typically fulfill such requirements.
For instance, in U.S. Pat. No. 4,770,183 (ADVANCED MAGNETICS), there is recommended to use biodegradable sub-micron sized superparamagnetic metal oxide particles (1-50 nm) which may be used uncoated or coated with a polysaccharide (like dextran) or serum albumin. Coating is effected by precipitating the particles with alkali, starting with water solutions of metal salts in the presence of the polymer. These products are suitable for intravenous applications as well as for gastro-intestinal applications, in which case they are administrable by intubation or enema, presumably because otherwise biodegradation by the stomach fluids would be too fast and toxicity might become a problem.
In WO85/04330 (NYCOMED), there is disclosed the use of ferromagnetic particles as contrast agents for NMRI. As mentioned before, ferromagnetic particles are bigger than superparamagnetic particles and behave as small permanent magnets which also achieve a significant reduction of T.sub.2. For direct administration into the digestive tract, the ferromagnetic particles are preferably embedded in a cellulose matrix or coated with this matrix. Cellulose derivatives can also be added as viscosants but the reference indicates that contrast enhancement is not readily achieved beyond the stomach, presumably because the embedding cellulose matrix does not protect sufficiently the particles from attack by the stomach fluids. Non-biodegradable embedding or coating matrices are therefore recommended to minimize absorption of toxic materials by the body.
EP-A-186.616 (SCHERING) discloses the use of complexes of particles of magnetite (Fe.sub.3 O.sub.4), .gamma.-iron oxide (Fe.sub.2 O.sub.3) and metal ferrites as contrast agents for NMRI. The cited complexants include oligo- and polysaccharides, proteins, polycarboxylic acids, protective colloids and other compounds. Examples of such compounds comprise polyvinyl-alcohol (PVA), polysilanes, polyethylene-imine, dextran, dextrin, oleic acid, gelatin, globulin, albumin, insulin, peptides and antibodies. The particles can also be encapsulated in liposomes. For enteral administration, the contrast agents are suspended in a water medium which may contain further ingredients such as salt or excipients like methylcellulose, viscosants, lactose, mannitol and surfactants like lecithin, Tween.RTM., Myrj.RTM. and the like.
For enteral use, this document particularly mentions compositions containing dextrin- or dextran-magnetite complexes, the manufacturing of which is disclosed in U.S. Pat. No. 4,101,435 (MEITO SANGYO).
There is also reported by J. KLAVENESS et al, in Diagnostic Imaging International, November 1988, p. 70, the use as contrast agents for the gastro-intestinal system of microspheres (3.5 .mu.m) of sulfonated ion-exchange styrene-divinylbenzene resin coated with magnetite Fe.sub.3 O.sub.4. The matrix is non-biodegradable and the particles with an iron content of about 20% are stable in the gastro-intestinal tract.
After testing the compositions of the cited prior art consisting of coated or uncoated magnetic particles in admixtures with polymer carriers, the present inventors noted that the contrast effect in NMR imaging is generally unstable and rapidly vanishes, presumably because despite the presence of the carrier phase the magnetic particles tend to coalesce or coacervate together under the influence of the external magnetic field which strongly reduces their controlling effect on the spin-relaxation of the neighbouring protons. They however also found that such undesirable coalescence of the magnetic particles and vanishing of the T.sub.2 relaxing effect can be prevented by either selecting as the carrier phase substantially water-insoluble hydrophilic water-swelling substrates which tend to form gels with water or, when using water-soluble polymer carriers, raising the pH of the aqueous medium containing the magnetic particles to at least 13 when admixing with the polymer solution, and keeping the dry weight ratio of said polymers to magnetic particles not below 5:1 rand, preferably, in the range of 100:1-10:1.
Although ;the exact reason of these findings is not definitely explained, it can be postulated that using carrier matrices which form nearly insoluble gels upon admixing with water (thixotropic or pseudo-plastic solutions) will locally raise the viscosity at the particle/carrier interface to such extent that the particle mobility is impeded and agglomeration is prevented.